The present invention relates to an amorphous silicon semiconductor and an amorphous silicon photovoltaic device.
Since it was found in 1976 by W. E. Spear et al that the conductivity of the amorphous silicon obtained by a plasma decomposition method of silane (SiH.sub.4) could be greatly altered by doping with phosphine (PH.sub.3) and diborane (B.sub.2 H.sub.6) and an amorphous silicon solar cell was fabricated on an experimental basis in 1976 by D. E. Carlson et al, the amorphous silicon solar cell has attracted attention, and many studies with the aim of improvement of its efficiency have been made.
The Schottky barrier type, PIN type, MIS type and heterojunction type are known structures of photovoltaic devices using an amorphous silicon thin film. The former three types promise to provide highly efficient solar cells. For instance, a Schottky barrier type solar cell made by D. E. Carlson et al in 1977 showed a conversion efficiency of 5.5%, an MIS type solar cell made by J. I. B. Wilson et al in 1978 showed a conversion efficiency of 4.8%, and a PIN junction solar cell made by Yoshihiro Hamakawa in 1978 showed a conversion efficiency of 4.5%.
In the case of the PIN junction solar cell, the P or N type amorphous silicon has a short carrier life time and, therefore, fails to provide effective carriers. Also, the P-layer suffers from light absorption loss because it has a higher light absorption coefficient than the I-layer. In order to eliminate these drawbacks, an inverted PIN junction photovoltaic device has been proposed. This photovoltaic device is constructed so that the light impinges on the N-type amorphous silicon side. Since this device has a smaller light absorption coefficient than the P-type, it is believed to be more advantageous, though slightly. Nevertheless this N-type amorphous silicon is no better than the P-type in the sense that it similarly suffers from light absorption loss.
Also, in the case of the PIN junction solar cell, it is necessary to form a transparent electrode on the light-impinging side, and ITO (indium-tin-oxide) (In.sub.2 O.sub.3 +SnO.sub.2) or SnO.sub.2 has been employed as a transparent electrode. However, cells with the ITO electrode have the disadvantage that while the fill factor is good, the open circuit voltage is low, and cells with the SnO.sub.2 electrode have the disadvantage that while the open circuit voltage is high, the fill factor is bad.
It is an object of the present invention to provide an amorphous silicon semiconductor suitable for use in fabricating an amorphous silicon PIN junction photovoltaic device.
Another object of the invention is to provide an amorphous silicon PIN junction photovoltaic device having an improved conversion efficiency.
It is a still further object to provide a two-layer transparent electrode on the light impinging side of an amorphous silicon type solar cell.
These and other objects of the present invention will become apparent from the description hereinafter.